Question: In a class of $9$, there are $7$ students who are secretly robots. If the teacher chooses $4$ students, what is the probability that all four of them are secretly robots?
We can think about this problem as the probability of $4$ events happening. The first event is the teacher choosing one student who is secretly a robot. The second event is the teacher choosing another student who is secretly a robot, given that the teacher already chose someone who is secretly a robot, and so on. The probabilty that the teacher will choose someone who is secretly a robot is the number of students who are secretly robots divided by the total number of students: $\dfrac{7} {9}$ Once the teacher's chosen one student, there are only $8$ left. There's also one fewer student who is secretly a robot, since the teacher isn't going to pick the same student twice. So, the probability that the teacher picks a second student who also is secretly a robot is $\dfrac{6} {8}$ The probability of the teacher picking two students who are secretly robots must then be $\dfrac{7} {9} \cdot \dfrac{6} {8}$ We can continue using the same logic for the rest of the students the teacher picks. So, the probability of the teacher picking $4$ students such that all of them are secretly robots is $\dfrac{7}{9}\cdot\dfrac{6}{8}\cdot\dfrac{5}{7}\cdot\dfrac{4}{6} = \dfrac{5}{18}$